Roll libvpx 269083:278497

See https://codereview.chromium.org/344903003/
for the actual changes.

BUG=386801
R=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/348683002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278772 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -41,7 +41,7 @@ vars = {
   "google_toolbox_for_mac_revision": "662",
   "libaddressinput_revision": "293",
   "libphonenumber_revision": "621",
-  "libvpx_revision": "269083",
+  "libvpx_revision": "278497",
   "lss_revision": "26",
   "sfntly_revision": "239",
   "lighttpd_revision": "33737",